I was fired at by mercs in one yesterday, so i sniped em and kept it. It's really cool to shell targets near the lake with this. Am i just ignorant, or is this quite rare??
There are a couple patrols in the south around the southern end of the lake that have grenade launchers on a boat. Plus one on the river near the rail xing bridge. Check out the maps and look for red patrol lines.
I used one in one of my raids on the Ranger Station, quite fun :)
There's a GL boat docked on the east side of the brewery - easy to steal, then make your way to the back side of the mortar merc island to off that pesky guy. Watch for the other GL and .50 boats though, they come up quickly and exert huge damage (as I'm sure all of you already know). Snipe the mercs in the boats (I typically encounter up to 4 gun boats) and then you have free reign over the southern shoreline - launch to your heart's content and wipe out everything that's alive (except bugs, of course)
Also one hidden in a cove to the southwest of the dental clinic, about midway between the Ranger Station, Clinic, and Post Office. I supposed it's supposed to go with the safe house, but it's good for shelling all those places, and a weapons convoy that goes through there.